# ERDOS PROBLEMS ON IRREGULARITIES OF LINE SIZES AND POINT DEGREES A. GYARFAS*

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1 BOLYAI SOCIETY MATHEMATICAL STUDIES, 11 Paul Erdos and his Mathematics. II, Budapest, 2002, pp ERDOS PROBLEMS ON IRREGULARITIES OF LINE SIZES AND POINT DEGREES A. GYARFAS* Problems and results are given concerning some variants on the irregularities of line sizes and point degrees. All questions are initiated by Paul Erdos. A linear space (Pairwaise balanced design) is a set of lines (edges, blocks) on n points (vertices, treatments) with the property that each pair of distinct points is covered by a unique line. The size of a line is the number of points on it and the degree d(p) of a point pis the number of lines containing it. A linear space is planar if its points can be placed in R 2 so that the lines are the maximal collinear subsets. A well-known example of a nonplanar linear space is the Farro plane. We shall assume that line sizes are between 2 and n - 1, i.e. singleton lines and trivial spaces are excluded. This note addresses irregularity problems asked by Paul Erdos: what is the maximum number of distinct line-sizes (point degrees) and what is the minimum number of repetitions for them. Here the maximum and minimum is taken over all linear spaces of n points. The four questions becomes eight since each has a planar variant. Similar questions of Erdos involving distances are not treated here, see [6] and its references. For problems and results concerning linear spaces in general see [7]. * Partly supported by grant OTKA T

2 368 A. Gyarfas 1. MAXIMUM NUMBER OF DISTINCT LINE SIZES: f1(n) One can state a surprisingly sharp result here. Erdos, Duke, Fowler and Phelps ([3]) prove that!1(n) = 2ln J- 2. There is a slight error in the proof, in fact!1 (n) is one larger for certain values of n. The correct value is stated in the next theorem. Theorem 1. For k 2 :::; n < k 2 + k, j 1 ( n) = 2k - 2 and for k 2 + k :::; n < (k + 1) 2, /1(n) = (2k- 1). Proof. The upper bound comes from the following Claim. Assume that e1, e2,..., et are lines in a linear space and I e1i < le2l <... < ietl Then Proof of Claim. For each i (1 :::; i :::; t- 1), ei+l intersects U~=l ej in at most i points and jei+ll 2: ie1i + i. Case 1, n < k 2 + k. Assume that k:::; le1i <... jetl Then, using the claim, k 2 + k > n 2: kt which gives k + 1 > t, i.e there are at most k distinct line sizes not smaller than k. Since at most k- 2 line sizes are in the interval [2, k - 1],!1 ( n) :::; k + k - 2 = 2k - 2. Case 2, n < (k + 1) 2. Assume that k + 1:::; je1i <... jetl Then, using the claim, (k + 1) 2 > n 2: (k + l)t which gives k + 1 > t, i.e there are at most k distinct line sizes not smaller than k + 1. Since at most k - 1 line sizes are in the interval [2, k],!1 (n) :::; k + k- 1 = 2k- 1. The lower bound of the theorem comes from the following construction. Construction 1. Consider lines e1,..., ek in general position and parallel segments /1,..., fk One can easily arrange these lines so that fi intersects precisely e1,..., ei and these intersection points are all distinct from the (;) intersection points of the ei-s. We have k 2 points with line sizes 2, 3,..., 2k- 1, i.e. 2k- 2 line sizes. For 1 :::; t :::; k- 1 it is trivial to extend the example with t new points not collinear to any lines ei, fi Therefore!1 (n) 2: 2k- 2 for k 2 :::; n < k 2 + k. To see that f 1 (n) 2: 2k - 1 for n = k 2 + k, add k new points to the previous construction, one to each ei, so that the new points are not collinear to any /j. Each ei is extended by one point so we have line sizes

3 Erdos Problems on Irregularities of Line Sizes and Point Degrees 369 k + 1, k + 2,..., 2k and the /j-s retain their sizes 2, 3,..., k thus we have 2k - 1 distinct line sizes. As before, it is trivial to extend this with t new points for 1 :::; t :::; k - 1. This finishes the proof of Theorem 1. Since the equality in Theorem 1 is shown by a planar example, we get the following corollary. Corollary 1. n points of the plane determine at most 2k - 2 (2k - 1) distinct line sizes if k 2 :::; n < k 2 + k (if k 2 + k :::; n < (k + 1) 2 ). This is best possible for every n. 2. MINIMUM NUMBER OF LINE SIZE REPETITIONS: f2(n) Erdos and Purdy introduced!2(n) in 1978 ([4]) as the minimum r such that every linear space on n points has a line size repeated at least r times. Conjecture 1 (Erdos-Purdy, [4]). j 2 (n) :::; cn where c is an absolute constant. In [4] an outline of a probabilistic argument is given to show that j 2 (n) :::; cn Their example is a linear space obtained from a projective plane by the intersections of its lines with a random subset of points. In fact, as pointed out by Blokhuis, Szonyi and Wilbrink ([1]), one can not improve the upper bound this way because among the line sizes defined by any set of at least c 1 q 2 points of a projective plane of order q there must be a line size repeated at least c2q 3 / 4 times. It is also a remark in [4] that the upper bound j 2 (n) 2:: cn follows easily from De Bruijn-Erdos theorem ([2]). Indeed, since there are less than 2ln J line sizes (Theorem 1) and at least n lines ([2]), some line size must be repeated at least n~ 2 times. Corollary 2.!2(n) > n~ 2 Conjecture 1 does not hold for the planar version of the problem. The reason is that a planar set of n points must determine at least 3 F Gallai lines (lines which contain precisely two points). In fact, Erdos and Purdy proved ([4]) more which gives that!2(n) = n- 1 for planar sets:

4 370 A. Gyarfas Theorem 2 ([4]). Assume that n 2: 25 points of the plane are not collinear. Then there are at least n - 1 Gallai lines or at least n - 1 three-point lines. 3. MAXIMUM NUMBER OF DISTINCT POINT DEGREES: j3(n) Problem 1 (Erdos and Makai, 1992). Estimate j3(n), the maximum number of distinct point degrees in a linear space of n points. How does j 3 ( n) change if restricted to planar linear spaces? Construction 2. The following construction shows that j3(n) 2: n/3 for planar sets. Let a, b, c denote three parallel lines such that the distance of a, b and the distance of b, c are the same. Select k points B1, B2,..., Bk on b so that the distances BiBi+l are the same. Select point C 1 on line c. The points Ai are defined as the intersection of a and the line C1Bi and the points Ci are defined as the intersection of c and the line A 1 Bi (1 ~ i ~ k). The set of point degrees is equal to {k + 1, k + 2,..., 2k} on lines a, b, c. Construction 2a. If line c in Construction 2 is replaced by (~) lines, one gets a nonplanar example showing that j3(n) 2: 2n/3. 2-point The upper bound is much weaker than the lower bounds provided by the above constructions: Theorem 3 (Erdos-Gyarfas unpublished). Proof. Let x be a point of minimum degree t with lines lr, l2,... lt through x, set llil = ni, n1 2: n2 2:... 2: nt. The set of n points is denoted by V. Claim. For y Eli, y i= x, n-n t _ 1 z < d (y) ~ n - ni + 1. Proof of Claim. The lines through y different from li intersect V \ li in at most t - 1 points. This gives the lower bound and the upper bound is obvious.

5 Erdos Problems on Irregularities of Line Sizes and Point Degrees 371 Using the claim we conclude that no point (except possibly x) has degree less than nt-:::_~ 1 If n1 ::; (1/2 + c)n for some > 0 then no point has degree less than n(1/2-c) n-n1 ~~---- <. t-1 - t-1 On the other hand, by the definition oft, no point has degree less than t. Therefore max {t- 1, n( 1 / 2 - c) - 1} t-1 'small' degrees are missing which is clearly at least ( n(1/2 - c)) 1 / 2. If n1 ~ (1/2 + c)n then (from the claim) each point of h (except x) has degree at most n- n1 + 1 ::; n(1/2- c)+ 1. On the other hand, each point y =1- x of any li fori =1-1 has degree at least n1 ~ n(1/2+c) because y and the points of h are on distinct lines. Therefore each degree (with the possible exception of d(x)) is missing from the interval [ n(1/2- c)+ 1, n(1/2 +c)] and the proof is finished. The following two problems had been formulated by Paul Erdos and the author. Problem 2. Is it true that j 3 (n) ~ n- cn 1 1 2? Problem 3. Is it true that for planar sets j3(n) < (1 - c)n for some positive c? 4. MINIMUM NUMBER OF DEGREE REPETITIONS: j4(n) Construction 2 gives a planar linear space where each point degree is repeated at most three times, i.e. j4(n) ::; 3. Problem 4. Is j 4 (n) ~ 3 for planar linear spaces or there exists a planar point set where each point degree is repeated at most twice? Notice that, like for degrees of a graph, some degrees must be repeated. It is well known that there are graphs in which each degree is repeated at most twice. The next construction shows that if planarity is not required then (like in the case of graphs) there are linear spaces in which each point degree is repeated at most twice.

6 372 A. Gyarfas Construction 3. We start with Construction 2 assuming that k is odd and k 2:': 7. After some modifications we arrive to L*, a linear space on 3k + 1 points, 2k - 2 distinct point degrees, each repeated at most twice. Line a is replaced by a near pencil with center Ak-I Line b is replaced by a near pencil with center BI Line cis replaced by (;) 2-point lines. The resulting linear space is denoted by L. Next the following partition P = {PI, P2,... Pk+9 } is defined on the 2 points of L. Set PI = a\ Ak-I, p2 = {Ak-I, BI}, p3 = b \ BI, pi = ci-3 fori= 4, 5, 6 and for 7:::; i:::; k! 9 set Pi= {C2i-Io, C2i-9} The linear space L* is formed by adding a new point D to L which form a line with each Pi. The blocks Pi which formed lines in L (all but P4, Ps, P6) are removed. The degree of D is k! 9 while the degrees of points in L do not change, except at CI, C2, C3 which are increased by one. The degree sequence can be divided into four intervals 1j as follows. h = [ k! 9 J repeated once; h = [k + 2, 2k + 2] repeated twice; 13 = [2k + 3, 3k - 4] repeated once; 14 = [3k - 3, 3k - 2] repeated twice. To ensure separation of these intervals, we need the conditions k! 9 < k+2 and 2k+2 < 3k-3 thus the construction works for odd k > 5. It is easy to check that adding one or two new points to Construction 3 with 2-point lines preserves the property that each degree is repeated at most twice. Corollary 3. For every n 2:': 22, ]4(n) = 2. REFERENCES [1] A. Blokhuis, T. Szonyi and H. A. Wilbrink, Personal communication. [2] N. G. de Bruijn and P. Erdos, On a combinatorial problem, Indagationes Math., 10 (1948), [3] P. Erdos, R. A. Duke, J. C. Fowler and K. T. Phelps, Extremal problems for Pairwise balanced designs, Congressus Numerantium, 48 (1985), [4] P. Erdos and G. Purdy, Some Combinatorial Problems in the Plane, J. of Combinatorial Theory A, 25 (1978), [5] L. Kelly and W. Moser, On the number of ordinary lines determined by n points, Canadian J. of Mathematics, 10 (1958),

7 Erdos Problems on Irregularities of Line Sizes and Point Degrees 373 [6] Gy. Csizmadia and D. Ismailescu, Maximum number of different distance counts, in: Bolyai Society Math. Studies, 6 (Intuitive Geometry, 1997), [7] K. Metsch, Linear spaces with few lines, Springer Lecture Notes in Mathematics No (Berlin, Heidelberg, New York, London, 1991). Andras Gyarfas Computer and Automation Research Institute Hungarian Academy of Sciences Kende u Budapest 1111 Hungary Gyarfas~sztaki.hu

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